1. Field of the Invention
The present invention relates to an internal combustion engine including a plasma generating device that generates plasma.
2. Description of Background Art
An internal combustion engine is known which includes a plasma generating device that converts plasma-generating gas into a plasma state by electrical discharge to generate plasma containing radicals, the plasma generating device injects plasma toward a combustion chamber from its distal end portion facing the combustion chamber. See, for example, JP-A No. 2006-316777 (FIG. 11).
It is known that when generating plasma by a plasma generating device, the lower the pressure within a generating chamber (hereinafter; referred to as “generating-atmosphere pressure”) where plasma is generated in the plasma generating device, the easier it is to generate plasma. Therefore, in the case of a plasma generating device used in an internal combustion engine, plasma generation efficiency can be improved if plasma can be generated at a pressure equivalent to a compression pressure within the combustion chamber near the compression top dead center corresponding to the timing of the air-fuel mixture ignition or the timing immediately before ignition, or at a generating-atmosphere pressure lower than a generating-atmosphere pressure equal to or higher than the compression pressure.
With an improvement in plasma generation efficiency, an improvement in combustibility due to radicals contained in plasma can be anticipated. Furthermore it is possible to reduce the size, weight, and power consumption of the plasma generating device.
Further, an internal combustion engine is known which includes a plasma generating device that uses exhaust gas as the plasma-generating gas wherein generated plasma is supplied to an intake passage. See, for example, JP-A No. 2004-340048.
Since plasma contains radicals, adding plasma to intake air or an air-fuel mixture improves combustibility. However, the intake rate of intake air to be sucked into the combustion chamber in accordance with a required load on the internal combustion engine varies greatly depending on the operational state of the internal combustion engine. Thus, if the amount of generated plasma is not sufficient relative to the intake rate, it is difficult to fully exploit the combustibility improving effect due to plasma across a wide operating range of the internal combustion engine. Further, there are also cases where the amount of plasma becomes excessive relative to the intake rate.
Accordingly, to control the amount of generated plasma, it may be possible to make the gas rate of the plasma-generating gas introduced into the plasma generating device constant for controlling the discharge voltage for generating electrical discharge for converting the plasma-generating gas into a plasma state. In this case, however, the presence of the plasma-generating gas that is not converted into plasma may make it difficult to adjust the mixing ratio (i.e., the air-fuel ratio) in an air-fuel mixture between fuel and air sucked into the combustion chamber to ensure the uniformity of the mixing.
In a case where an intake pressure in the intake passage is used to introduce plasma-generating gas to the reaction chamber, since the opening of the throttle valve is small during low load operation of the internal combustion engine, the intake pressure downstream of the throttle valve becomes low (the negative pressure becomes large), so the gas rate of plasma-generating gas tends to become excessive. On the other hand, since the opening of the throttle valve becomes large during high load operation of the internal combustion engine, and the intake pressure downstream of the throttle valve becomes high (the negative pressure becomes small), the gas rate of plasma-generating gas becomes insufficient, so the combustibility improving effect due to plasma may not be fully attained.